A new technique for producing coatings on a wide variety of substrate surfaces by kinetic spray, or cold gas dynamic spray, was recently reported in an article by T. H. Van Steenkiste et al., entitled “Kinetic Spray Coatings,” published in Surface and Coatings Technology, vol. 111, pages 62-71, Jan. 10, 1999. The article discusses producing continuous layer coatings having low porosity, high adhesion, low oxide content and low thermal stress. The article describes coatings being produced by entraining metal powders in an accelerated air stream, through a converging-diverging de Laval type nozzle and projecting them against a target substrate. The particles are accelerated in the high velocity air stream by the drag effect. The air used can be any of a variety of gases including air or helium. It was found that the particles that formed the coating did not melt or thermally soften prior to impingement onto the substrate. It is theorized that the particles adhere to the substrate when their kinetic energy is converted to a sufficient level of thermal and mechanical deformation. Thus, it is believed that the particle velocity must be high enough to exceed the yield stress of the particle to permit it to adhere when it strikes the substrate. It was found that the deposition efficiency of a given particle mixture was increased as the inlet air temperature was increased. Increasing the inlet air temperature decreases its density and increases its velocity. The velocity varies approximately as the square root of the inlet air temperature. The actual mechanism of bonding of the particles to the substrate surface is not fully unknown at this time. It is believed that the particles must exceed a critical velocity prior to their being able to bond to the substrate. The critical velocity is dependent not only on the material of the particle but also on the size of the particle. It is believed that the initial particles to adhere to a substrate have broken the oxide shell on the substrate material permitting subsequent metal to metal bond formation between plastically deformed particles and the substrate. Once an initial layer of particles has been formed on a substrate subsequent particles bind not only to the voids between previous particles bound to the substrate but also engage in particle to particle bonds. The bonding process is not due to melting of the particles in the air stream because the temperature of the air stream is always below the melting temperature of the particles and the temperature of the particles is always below that of the air stream.
This work improved upon earlier work by Alkimov et al. as disclosed in U.S. Pat. No. 5,302,414, issued Apr. 12, 1994. Alkimov et al. disclosed producing dense continuous layer coatings with powder particles having a particle size of from 1 to 50 microns using a supersonic spray.
The Van Steenkiste article reported on work conducted by the National Center for Manufacturing Sciences (NCMS) to improve on the earlier Alkimov process and apparatus. Van Steenkiste et al. demonstrated that Alkimov's apparatus and process could be modified to produce kinetic spray coatings using particle sizes of greater than 50 microns and up to about 106 microns.
This modified process and apparatus for producing such larger particle size kinetic spray continuous layer coatings are disclosed in U.S. Pat. Nos. 6,139,913, and 6,283,386. The process and apparatus provide for heating a high pressure air flow up to about 650° C. and combining this with a flow of particles. The heated air and particles are directed through a de Laval-type nozzle to produce a particle exit velocity of between about 300 m/s (meters per second) to about 1000 m/s. The thus accelerated particles are directed toward and impact upon a target substrate with sufficient kinetic energy to impinge the particles to the surface of the substrate. The temperatures and pressures used are sufficiently lower than that necessary to cause particle melting or thermal softening of the selected particle. Therefore, no phase transition occurs in the particles prior to impingement. It has been found that each type of particle material has a threshold critical velocity that must be exceeded before the material begins to adhere to the substrate.
One difficulty associated with all of these prior art kinetic spray systems arises from the configuration of the de Laval type nozzle. These converging-diverging nozzles typically converge from a diameter of approximately 7.0 to 10.0 mm down to a throat of from 2.0 to 3.0 mm and then diverge into a variety of shapes including rectangular openings of from 2.0 to 5.0 mm by 10.0 to 30.0 mm. The very narrow throat diameters cause the nozzles to plug very rapidly, requiring a shut down of the system and unplugging of the nozzle. Many times, depending on the particle material, gas temperature and velocity the nozzles may plug in as short as 1 minute or less. Each type of particle material has a threshold critical velocity at which it will start to adhere to the interior of the nozzle. The critical velocity is dependent on both the particle size and its material composition. The surfaces inside the nozzle must be kept free of obstructions to enable proper coating. Partial plugging is also a problem because the coated surface may appear to be good, however, internal defects will result in poor mechanical properties. Clearly, this severely limits the practical usefulness of the method. Thus, it would be highly desirable to provide a system and method to greatly reduce or eliminate this problem. It would also be highly beneficial to raise the temperature of the main gas while preventing plugging as this increases the deposition efficiency.